1. Field of the Invention
This invention relates to phosphate ester base stock compositions comprising mixed n-butyl/isobutyl phosphate esters and to aircraft hydraulic fluid compositions comprising such base stocks.
2. State of the Art
Hydraulic fluids used in the hydraulic systems of aircraft must meet exacting specifications set by aircraft manufacturers. Accordingly, the components of aircraft hydraulic fluids are carefully chosen to balance, among other properties, stability, compatibility, density, toxicity and the like. Whether the selected components can, in fact, be balanced to meet these specifications is unpredictable. Moreover, the amounts of individual components used in compositions which meet the specifications is not a priori predictable.
Trialkyl phosphate esters, such as tri-n-butyl phosphate and triisobutyl phosphate, have been used previously as base stocks for aviation hydraulic fluids. For example, trialkyl phosphate ester base stocks are described in U.S. Pat. No. 5,464,551, the disclosure of which is incorporated herein by reference in its entirety. In particular, low density aviation hydraulic fluids, i.e., fluids having a density below about 1.020 g/L at 25.degree. C., have conventionally been prepared using tri-n-butyl phosphate as the major component of the base stock. However, tri-n-butyl phosphate is known to be a skin irritant and minimizing its concentration is desirable. Alternatively, low density fluids employing triisobutyl phosphate as the major component have had difficulty meeting the low volatility and low temperature viscosity requirements imposed on aviation hydraulic fluids.
It has now been discovered that phosphate ester base stocks comprising mixed isobutyl/n-butyl phosphate esters, i.e., n-butyl diisobutyl phosphate or di-n-butyl isobutyl phosphate or mixtures thereof, have surprising and unexpected properties when compared to base stocks containing major amounts of tri-n-butyl phosphate and triisobutyl phosphate or physical mixtures thereof. Specifically, it has been found that by employing mixed isobutyl/n-butyl phosphate esters in the base stock of the fluid, an unexpected, surprising balance of properties critical to aviation hydraulic fluids is obtained, including acceptable hydrolytic stability, high flash point, good anti-wear properties, acceptable erosion protection, acceptable low temperature flow properties (viscosity), and elastomer compatibility.